In recent years, ceramic multilayer circuit boards mainly employ low-temperature fired boards and are used as small components such as for personal computers and cellular phones. When manufacturing these ceramic multilayer boards, it is general practice to laminate ceramic green sheets. Screen printing method is generally employed in forming inner-layer wiring conductors while screen printing method, thin-film photolithography, or thick-film photolithography (for example, Fodel by DuPont) is employed in forming outer-layer wiring conductors.
In extremely miniaturized electronic components to be used in devices such as cellular phones, printed circuit boards have built-in inductor elements (L), capacitor elements (C) or resistor elements (R). This has enabled development of circuit boards having an L-C-R combined circuit such as a filter. As L-C-R elements and inner wiring conductors are formed by screen printing as disclosed in Japanese Patent Application Non-Examined Publication No. H02-310996, for example, there has been a need for a technology to make screen printing finer. However, in screen printing, 100 μm is the limit of wiring pitch (line width plus interwiring distance) in manufacturing.
Referring to FIG. 3A to FIG. 3E, a description will be given on conventional technology for manufacturing green sheet multilayer circuit boards. As illustrated in FIG. 3A, through holes 32 are first made on ceramic green sheet 31 by means of a punching device, stamping die, or YAG laser device. Subsequently, via hole conductors 33 are formed by screen printing conductive paste in through holes 32 as illustrated in FIG. 3B. Next, as illustrated in FIG. 3C, wiring conductors 34 are formed by screen printing conductive paste on a green sheet on which via hole conductors 33 have been formed. Furthermore, two or more green sheets on which the above-mentioned wiring conductors 34 have been formed are laminated and integrated into one piece by hot pressing as illustrated in FIG. 3D. The binder in the work is then burned-out and fired to obtain a ceramic multilayer circuit board. In the conventional method of manufacture, as the inner conductor pattern is made by screen printing, the wiring pitch has a limit of the order of 100 μm and the thickness of the conductive film after firing is of the order of 7–8 μm.
In order to make the packing density of a ceramic circuit board high, it is necessary to employ thick-film intaglio transfer process for the formation of the conductor patterns as the thick-film intaglio transfer process can form conductor patterns having fineness (wiring pitch of the order of 40 μm) and high aspect ratio (fired thickness of the order of 10 μm). If conductor patterns can be formed on a ceramic green sheet by thick-film intaglio transfer process, then it becomes possible to manufacture ceramic multilayer circuit boards by applying the technique to laminated green sheets. Such a method is disclosed in Japanese Patent Application Non-Examined Publication No. H11-121645, for example.
In order to directly transfer conductor patterns on a ceramic green sheet by thick-film intaglio transfer process, it is necessary to coat on the ceramic green sheet an adhesive layer having thermoplastic resin as the main ingredient. However, as the solution of the adhesive layer contains high volatility solvent such as toluene and acetone, ceramic green sheet dissolves in the solution. Also, even though an adhesive layer could have been formed on the surface of a ceramic green sheet, when an intaglio filled with conductive paste and the ceramic green sheet come into close contact with an adhesive interposed in between after hot pressing, the ceramic green sheet undergoes internal fracture during the process of peeling the intaglio as the ceramic green sheet is spongy.